This action funds an NSF Postdoctoral Research Fellowship in Biology for FY 2025. The fellowship supports research and training of the fellow that will contribute to biology in innovative ways. The marine subsurface biosphere is the least explored biome on Earth. Studying microbes that live in this vast habitat enhances our understanding of global biogeochemical cycles and can support efforts to store excess carbon in seafloor sediment. Mounting evidence suggests eukaryotes are ecologically important in marine sediments, yet limited sampling of subseafloor eukaryotes has made it difficult to estimate their contributions to nutrient cycling and their potential to sequester sunken organic matter (OM) into the sediment. Investigating these microbes is critical as the composition of subsurface sediment changes. Increasing terrestrial runoff is adding more land-derived OM to marine sediments, and microbes’ response to inputs of different origin may impact the global carbon cycle. To address these questions, the fellow will analyze microbial communities in sediment cores collected along a transect from the shallow Puerto Rico continental shelf to the depths of the Atlantic Ocean. The results will provide critical data needed to identify which seafloor zones are most likely to permanently store sunken carbon. Undergraduate and high school students will be mentored throughout the project, and a new museum exhibit will be created to share these discoveries with the public at the Woods